Network Working Group R. van Brandenburg
Internet-Draft O. van Deventer
Intended status: Informational TNO
Expires: October 14, 2013 F. Le Faucheur
K. Leung
Cisco Systems
April 12, 2013
Models for adaptive-streaming-aware CDN Interconnectiondraft-brandenburg-cdni-has-05
Abstract
This documents presents thoughts on the potential impact of
supporting HTTP Adaptive Streaming technologies in CDN
Interconnection (CDNI) scenarios. The intent is to present the
authors' analysis of the CDNI-HAS problem space and discuss different
options put forward both by the authors (and by others during
informal discussions) on how to deal with HAS in the context of CDNI.
THis document has been used as input information during the WG
process for making its decision regarding support for HAS.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 14, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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This document presents a discussion on what the impact of these
different characteristics is to the CDNI interfaces and what HAS-
specific optimizations may be required or may be desirable. The
scope of this document is to present the authors' analysis of the
CDNI-HAS problem space and discuss different options put forward both
by the authors (and by others during informal discussions) on how to
deal with HAS in the context of CDNI. The documents concludes by
presenting the authors' recommendations on how the CDNI WG should
deal with HAS in its initial charter, with a focus on 'making it
work' instead of including 'nice-to-have' optimizations that might
delay the development of the CDNI WG deliverables identified in its
initial charter.
It should be noted that the document is not a WG document, but has
been used as input information during the WG process for making its
decision regarding support for HAS. We expect the analysis presented
in the document will also be useful in the future if and when the WG
re-charters and wants to re-assess the level of HAS optimizations to
be supported in CDNI scenarios.
1.1. Terminology
This document uses the terminology defined in [RFC6707] and
[I-D.ietf-cdni-framework].
For convenience, the definition of HAS-related terms are restated
here:
Content Item: A uniquely addressable content element in a CDN. A
content item is defined by the fact that it has its own Content
Metadata associated with it. It is the object of a request routing
operation in a CDN. An example of a Content Item is a video file/
stream, an audio file/stream or an image file.
Chunk: a fixed length element that is the result of a segmentation or
fragmentation operation and that is independently addressable.
Fragment: A specific form of chunk (see Section 2.1). A fragment is
stored as part of a larger file that includes all chunks that are
part of the Chunk Collection.
Segment: A specific form of chunk (see Section 2.1). A segment is
stored as a single file from a file system perspective.
Original Content: Non-chunked content that is the basis for a
segmentation of fragmentation operation. Based on Original Content,
multiple alternative representations (using different encoding
methods, supporting different resolutions and/or targeting different
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bitrates) may be derived, each of which may be fragmented or
segmented.
Chunk Collection: The set of all chunks that are the result of a
single segmentation or fragmentation operation being performed on a
single representation of the Original Content. A Chunk Collection is
described in a Manifest File.
Content Collection: The set of all Chunk Collections that are derived
from the same Original Content. A Content Collection may consist of
multiple Chunk Collections, each corresponding to a single
representation of the Original Content. A Content Collection may be
described by one or more Manifest Files.
Manifest File: A Manifest File, also referred to as Media
Presentation Description (MPD) file, is a file that list the way the
content has been chunked (possibly for multiple encodings) and where
the various chunks are located (in the case of segments) or how they
can be addressed (in the case of fragments).
2. HTTP Adaptive Streaming aspects relevant to CDNI
In the last couple of years, a wide variety of HAS-like protocols
have emerged. Among them are proprietary solutions such as Apple's
HTTP Live Streaming (HLS), Microsoft's HTTP Smooth Streaming (HSS)
and Adobe's HTTP Dynamic Streaming (HDS), and various standardized
solutions such as 3GPP Adaptive HTTP Streaming (AHS) and MPEG Dynamic
Adaptive Streaming over HTTP (DASH). While all of these technologies
share a common set of features, each has its own defining elements.
This chapter will look at some of the common characteristics and some
of the differences between these technologies and how those might be
relevant to CDNI. In particular, Section 2.1 will describe the
various methods to store HAS content and Section 2.2 will list three
methods that are used to address HAS content in a CDN. After these
generic HAS aspects are discussed, two special situations that need
to be taken into account when discussing HAS are addressed:
Section 2.3 discusses the differences between Live and VoD content,
while Section 2.4 discusses the scenario where multiple streams are
combined in a single Manifest File (e.g. for ad insertion purposes).
2.1. Segmentation versus Fragmentation
All HAS implementations are based around a concept referred to as
chunking: the concept of having a server split content up in numerous
fixed duration chunks, which are independently decodable. By
sequentially requesting and receiving chunks, a client can recreate
and play out the content. An advantage of this mechanism is that it
allows a client to seamlessly switch between different encodings of
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the same Original Content at chunk boundaries. Before requesting a
particular chunk, a client can choose between multiple alternative
encodings of the same chunk, irrespective of the encoding of the
chunks it has requested earlier.
While every HAS implementation uses some form of chunking, not all
implementations store the resulting chunks in the same way. In
general, there are two distinct methods of performing chunking and
storing the results: segmentation and fragmentation.
- With segmentation, which is for example mandatory in all versions
of Apple's HLS prior to version 7, the chunks, in this case also
referred to as segments, are stored completely independent from
each other, with each segment being stored as a separate file from
a file system perspective. This means that each segment has its
own unique URL with which it can be retrieved.
- With fragmentation (or virtual segmentation), which is for example
used in Microsoft's Smooth Streaming, all chunks, or fragments,
belonging to the same Chunk Collection are stored together, as
part of a single file. While there are a number of container
formats which allow for storing this type of chunked content,
Fragmented MP4 is most commonly used. With fragmentation, a
specific chunk is addressable by subfixing the common file URL
with an identifier uniquely identifying the chunk that one is
interested in, either by timestamp, by byterange, or in some other
way.
While one can argue about the merits of each of these two different
methods of handling chunks, both have their advantages and drawbacks
in a CDN environment. For example, fragmentation is often regarded
as a method that introduces less overhead, both from a storage and
processing perspective. Segmentation on the other hand, is regarded
as being more flexible and easier to cache. In practice, current HAS
implementations increasingly support both methods.
2.2. Addressing chunks
In order for a client to request chunks, either in the form of
segments or in the form of fragments, it needs to know how the
content has been chunked and where to find the chunks. For this
purpose, most HAS protocols use a concept that is often referred to
as a Manifest File (also known as Media Presentation Description, or
MPD); i.e. a file that lists the way the content has been chunked
and where the various chunks are located (in the case of segments) or
how they can be addressed (in the case of fragments). A Manifest
File, or set of Manifest Files, may also identify the different
representations, and thus Chunk Collections, available for a content.
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In general, a HAS client will first request and receive a Manifest
File, and then, after parsing the information in the Manifest File,
proceed with sequentially requesting the chunks listed in the
Manifest File. Each HAS implementation has its own Manifest File
format and even within a particular format there are different
methods available to specify the location of a chunk.
Of course managing the location of files is a core aspect of every
CDN, and each CDN will have its own method of doing so. Some CDNs
may be purely cache-based, with no higher-level knowledge of where
each file resides at each instant in time. Other CDNs may have
dedicated management nodes which, at each instant in time, do know at
which servers each file resides. The CDNI interfaces designed in the
CDNI WG will probably need to be agnostic to these kinds of CDN-
internal architecture decisions. In the case of HAS there is a
strict relationship between the location of the content in the CDN
(in this case chunks) and the content itself (the locations specified
in the Manifest File). It is therefore useful to have an
understanding of the different methods in use in CDNs today for
specifying chunk locations in Manifest Files. The different methods
for doing so are described in sections 2.2.1 to 2.2.3.
Although these sections are especially relevant for segmented
content, due to its inherent distributed nature, the discussed
methods are also applicable to fragmented content. Furthermore, it
should be noted that the methods detailed below for specifying
locations of content items in Manifest Files do not only relate to
temporally segmented content (e.g. segments and fragments), but are
also relevant in situations where content is made available in
multiple representations (e.g., in different qualities, encoding
methods, resolutions and/or bitrates). In this case the content
consists of multiple chunk collections, which may be described by
either a single Manifest File or multiple interrelated Manifest
Files. In the latter case, there may be a high-level Manifest File
describing the various available bitrates, with URLs pointing to
separate Manifest Files describing the details of each specific
bitrate. For specifying the locations of the other Manifest Files,
the same methods apply that are used for specifying chunk locations.
One final note relates to the delivery of the Manifest Files
themselves. While in most situations the delivery of both the
Manifest File and the chunks are handled by the CDN, there are
scenarios imaginable in which the Manifest File is delivered by e.g.
the Content Provider, and the Manifest File is therefore not visible
to the CDN.
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One method for specifying chunk locations in a Manifest File is
through the use of relative URLs. A relative URL is a URL that does
not include the HOST part of a URL but only includes (part of) the
PATH part of a URL. In practice, a relative URL is used by the
client as being relative to the location where the Manifest File has
been acquired from. In these cases a relative URL will take the form
of a string that has to be appended to the location of the Manifest
File to get the location of a specific chunk. This means that in the
case a Manifest File with relative URLs is used, all chunks will be
delivered by the same surrogate that delivered the Manifest File. A
relative URL will therefore not include a hostname.
For example, in the case a Manifest File has been requested (and
received) from:
http://surrogate.server.cdn.example.com/content_1/manifest.xml
, a relative URL pointing to a specific segment referenced in the
Manifest File might be:
segments/segment1_1.ts
Which means that the client should take the location of the Manifest
File and append the relative URL. In this case, the segment would
then be requested from http://surrogate.server.cdn.example.com/content_1/segments/segment1_1.ts
The downside of using relative URLs is that it forces a CDN to
deliver all segments belonging to a given content item with the same
surrogate that delivered the Manifest File for that content item,
which results in limited flexibility. Another drawback is that
Relative URLs do not allow for fallback URLs; should the surrogate
that delivered the Manifest File break down, the client is no longer
able to request chunks. The advantage of relative URLs is that it is
very easy to transfer content between different surrogates and even
CDNs.
2.2.2. Absolute URLs with Redirection
Another method for specifying locations of chunks (or other Manifest
Files) in a Manifest File is through the use of an absolute URL. An
absolute URL contains a fully formed URL (i.e. the client does not
have to calculate the URL as in the case of the relative URL but can
use the URL from the Manifest File directly).
In the context of Manifest Files, there are two types of absolute
URLs imaginable: Absolute URLs with Redirection and Absolute URLs
without Redirection. The two methods differ in whether the URL
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points to a request routing node which will redirect the client to a
surrogate (Absolute URL with Redirection) or point directly to a
surrogate hosting the requested content (Absolute URL without
Redirection).
In the case of Absolute URLs with Redirection, a request for a chunk
is handled by the request routing system of a CDN just as if it were
a standalone (non-HAS) content request, which might include looking
up the surrogate (and/or CDN) best suited for delivering the
requested chunk to the particular user and sending an HTTP redirect
to the user with the URL pointing to the requested chunk on the
specified surrogate (and/or CDN), or a DNS response pointing to the
specific surrogate.
An example of an Absolute URL with Redirection might look as follows:
http://requestrouting.cdn.example.com/
content_request?content=content_1&segment=segment1_1.ts
As can be seen from this example URL, the URL includes a pointer to a
general CDN request routing function and includes some arguments
identifying the requested segment.
The advantage of using Absolute URLs with Redirection is that it
allows for maximum flexibility (since chunks can be distributed
across surrogates and CDN in any imaginable way) without having to
modify the Manifest File every time one or more chunks are moved (as
is the case when Absolute URLs without Redirection are used). The
downside of this method is that it can adds significant load to a CDN
request routing system, since it has to perform a redirect every time
a client requests a new chunk.
2.2.3. Absolute URL without Redirection
In the case of the Absolute URL without Redirection, the URL points
directly to the specific chunk on the actual surrogate that will
deliver the requested chunk to the client. In other words, there
will be no HTTP redirection operation taking place between the client
requesting the chunk and the chunk being delivered to the client by
the surrogate.
An example of an Absolute URLs without Redirection is the following:
http://surrogate.cdn.example.com/content_1/segments/segment1_1.ts
As can be seen from this example URL, the URL includes both the
identifier of the requested segment (in this case segment1_1.ts), as
well as the server that is expected to deliver the segment (in this
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case surrogate.cdn.example.com). With this, the client has enough
information to directly request the specific segment from the
specified surrogate.
The advantage of using Absolute URLs without Redirection is that it
allows more flexibility compared to using Relative URLs (since
segments do not necessarily have to be delivered by the same server)
while not requiring per-segment redirection (which would add
significant load to the node doing the redirection). The drawback of
Absolute URLs without Redirection is that it requires a modification
of the Manifest File every time content is moved to a different
location (either within a CDN or across CDNs).
2.3. Live vs. VoD
Though the formats and addresses of Manifest Files and chunk files do
not typically differ significantly between live and Video-on-Demand
(VoD) content, the time at which the Manifest Files and chunk files
become available does differ significantly. For live content, chunk
files and their corresponding Manifest Files are created and
delivered in real-time. This poses a number of potential issues for
HAS optimization:
- With live content, chunk files are made available in real-time.
This limits the applicability of bundling for content acquisition
purposes. Prepositioning may still be employed, however, any
significant latency in the prepositioning may diminish the value
of prepositioning if a client requests the chunk prior to
prepositioning, or if the prepositioning request is serviced after
the chunk playout time has passed.
- In the case of live content, Manifest Files must be updated for
each chunk and therefore must be retrieved by the client prior to
each chunk request. Any Manifest-File based optimization schemes
must therefore be prepared to optimize on a per-segment request
basis. Manifest Files may also be polled multiple times prior to
the actual availability of the next chunk.
- Since live Manifest Files are updated as each new chunk becomes
available, the cacheability of Manifest Files is limited. Though
timestamping and reasonable TTLs can improve delivery performance,
timely replication and delivery of updated Manifest Files is
critical to ensuring uninterrupted playback.
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- Manifest Files are typically updated after the corresponding chunk
is available for delivery, to prevent premature requests for
chunks which are not yet available. HAS optimization approaches
which employ dynamic Manifest File generation must be synchronized
with chunk creation to prevent playback errors.
2.4. Stream splicing
Stream splicing is used to create media mashups, combining content
from multiple sources. A common example in which content resides
outside the CDNs is with advertisement insertion, for both VoD and
live streams. Manifest Files which contain Absolute URLs with
redirection may contain chunk or nested Manifest File URLs which
point to content not delivered via any of the interconnected CDNs.
Furthermore, client and downstream proxy devices may depend on non-
URL information provided in the Manifest File (e.g., comments or
custom tags) for performing stream splicing. This often occurs
outside the scope of the interconnected CDNs. HAS optimization
schemes which employ dynamic Manifest File generation or rewriting
must be cognizant of chunk URLs, nested Manifest File URLs, and other
metadata which should not be modified or removed. Improper
modification of these URLs or other metadata may cause playback
interruptions, and in the case of unplayed advertisements, may result
in loss of revenue for content providers.
3. Possible HAS Optimizations
In the previous chapter, some of the unique properties of HAS have
been discussed. Furthermore, some of the CDN-specific design
decisions with regards to addressing chunks have been detailed. In
this chapter, the impact of supporting HAS in CDN Interconnection
scenarios will be discussed.
There are a number of topics, or problem areas, that are of
particular interest when considering the combination of HAS and CDNI.
For each of these problem areas it holds that there are a number of
different ways in which the CDNI Interfaces can deal with them. In
general it can be said that each problem area can either be solved in
a way that minimizes the amount of HAS-specific changes to the CDNI
Interfaces or in way that maximizes the flexibility and efficiency
with which the CDNI Interfaces can deliver HAS content. The goal for
the CDNI WG should probably be to try to find the middle ground
between these two extremes and try to come up with solutions that
optimize the balance between efficiency and additional complexity.
In order to allow the WG to make this decision, this chapter will
briefly describe each of the following problem areas together with a
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number of different options for dealing with them. Section 3.1 will
discuss the problem of how to deal with file management of groups of
files, or Content Collections. Section 3.2 will deal with a related
topic: how to do content acquisition of Content Collections between
the uCDN and dCDN. After that, Section 3.3 describes the various
options for the request routing of HAS content, particularly related
to Manifest Files. Section 3.4 talks about a number of possible
optimizations for the logging of HAS content, while Section 3.5
discusses the options regarding URL signing. Section 3.6 finally,
describes different scenarios for dealing with the removal of HAS
content from CDNs.
3.1. File Management and Content Collections3.1.1. General Remarks
One of the unique properties of HAS content is that it does not
consist of a single file or stream but of multiple interrelated files
(segment, fragments and/or Manifest Files). In this document this
group of files is also referred to as a Content Collection. Another
important aspect is the difference between segments and fragments
(see Section 2.1).
Irrespective of whether segments or fragments are used, different
CDNs might handle Content Collections differently from a file
management perspective. For example, some CDNs might handle all
files belonging to a Content Collection as individual files, which
are stored independently from each other. An advantage of this
approach is that makes it easy to cache individual chunks. Other
CDNs might store all fragments belonging to a Content Collection in a
bundle, as if they were a single file (e.g. by using a fragmented
MP4 container). The advantage of this approach is that it reduces
file management overhead.
This section will look at the various ways with which the CDNI
interfaces might deal with these differences in handling Content
Collections from a file management perspective. The different
options can be distinguished based on the level of HAS-awareness they
require on the part of the different CDNs and the CDNI interfaces.
3.1.2. Candidate approaches3.1.2.1. Option 1.1: No HAS awareness
This first option assumes no HAS awareness in both the involved CDNs
and the CDNI Interfaces. This means that the uCDN uses individual
files and the dCDN is not explicitly made aware of the relationship
between chunks and it doesn't know which files are part of the same
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Content Collection. In practice this scenario would mean that the
file management method used by the uCDN is simply imposed on the dCDN
as well.
This scenario also means that it is not possible for the dCDN to use
any form of file bundling, such as the single-file mechanism which
can be to store fragmented content as a single file (see
Section 2.1). The one exception to this rule is the situation where
the content is fragmented and the Manifest Files on the uCDN contains
byte range requests, in which case the dCDN might be able to acquire
fragmented content as a single file (see Section 3.2.2.2).
Effect on CDNI interfaces:
o None
Advantages/Drawbacks:
+ No HAS awareness necessary in CDNs, no changes to CDNI Interfaces
necessary
- The dCDN is forced to store chunks as individual files.
3.1.2.2. Option 1.2: Allow single file storage of fragmented content
In some cases, the dCDN might prefer to store fragmented content as a
single file on its surrogates to reduce file management overhead. In
order to do so, it needs to be able to either acquire the content as
a single file (see Section 3.2.2.2), or merge the different chunks
together and place them in the same container (e.g. fragmented MP4).
The downside of this is that in order to do so, the dCDN needs to be
fully HAS aware.
Effect on CDNI interfaces:
o CDNI Metadata Interface: Add fields for indicating the particular
type of HAS (e.g. MPEG DASH or HLS) that is used and whether
segments or fragments are used
o CDNI Metadata Interface: Add field for indicating the name and
type of the Manifest File(s)
Advantages/Drawbacks:
+ Allows dCDN to store fragmented content as a single file, reducing
file management overhead
- Complex operation, requiring dCDN to be fully HAS aware
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Internet-Draft HTTP Adaptive streaming and CDNI April 20133.1.2.3. Option 1.3: Access correlation hint
An intermediary approach between the two extremes detailed in the
previous two sections is one that uses a 'Access Correlation Hint'.
This hint, which is added to the CDNI Metadata of all chunks of a
particular Content Collection, indicates that those files are likely
to be requested in a short time window from each other. This
information can help a dCDN to implement local file storage
optimizations for VoD items (e.g. by bundling all files with the
same Access Correlation Hint value in a single bundle/file), thereby
reducing the number of files it has to manage while not requiring any
HAS awareness.
Effect on CDNI interfaces:
o CDNI Metadata Interface: Add field for indicating Access
Correlation Hint
Advantages/Drawbacks:
+ Allows dCDN to perform file management optimization
+ Does not require any HAS awareness
+ Very small impact on CDNI Interfaces
- Expected benefit compared with Option 1.1 is small
3.1.3. Recommendation
Based on the listed pros and cons, the authors recommend the WG to go
for Option 1.1, the 'Do Nothing'-approach. The likely benefits from
going for Option 1.3 are not believed to be significant enough to
warrant changing the CDNI Metadata Interface. Although Option 1.2
would bring definite benefits for HAS aware dCDNs, going for this
options would require significant CDNI extensions that would impact
the WG's milestones. The authors therefore don't recommend to
include it in the current work but mark it as a possible candidate
for rechartering once the initial CDNI solution is completed.
3.2. Content Acquisition of Content Collections3.2.1. General Remarks
In the previous section the relationship between file management and
HAS in a CDNI scenario has been discussed. This section will discuss
a related topic, which is content acquisition between two CDNs.
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With regards to content acquisition, it is important to note the
difference between CDNs that do Dynamic Acquisition of content and
CDNs that perform Content Pre-positioning. In the case of dynamic
acquisition, a CDN only requests a particular content item when a
cache-miss occurs. In the case of pre-positioning, a CDN pro-
actively places content items on the nodes on which it expects
traffic for that particular content item. For each of these types of
CDNs, there might be a benefit in being HAS aware. For example, in
the case of dynamic acquisition, being HAS aware means that after a
cache miss for a giving chunk occurs, that node might not only
acquire the requested chunk, but might also acquire some related
chunks that are expected to be requested in the near future. In the
case of pre-positioning, similar benefits can be had.
3.2.2. Candidate Approaches3.2.2.1. Option 2.1: No HAS awareness
This first option assumes no HAS awareness in both the involved CDNs
and the CDNI Interfaces. Just as with Option 1.1 discussed in the
previous section with regards to file management, having no HAS
awareness means that the dCDN is not aware of the relationship
between chunks. In the case of content acquisition, this means that
each and every file belonging to a Content Collection will have to be
individually acquired from the uCDN by the dCDN. The exception to
the rule is in cases with fragmented content where the uCDN uses
Manifest Files which contain byte range requests. In this case the
dCDN can simply omit the byte range identifier and acquire the
complete file.
The advantage of this approach is that it is highly flexible. If a
client only requests a small portion of the chunks belonging to a
particular Content Collection, the dCDN only has to acquire those
chunks from the uCDN, saving both bandwidth and storage capacity.
The downside of acquiring content on a per-chunk basis is that it
creates more transaction overhead between the dCDN and uCDN compared
to a method in which entire Content Collections can be acquired as
part of one transaction.
Effect on CDNI interfaces:
o None
Advantages/Drawbacks:
+ Per-chunk content acquisition allows for high level of flexibility
between dCDN and uCDN
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- Per-chunk content acquisition creates more transaction overhead
between dCDN and uCDN
3.2.2.2. Option 2.2: Allow single file acquisition of fragmented content
As discussed in Section 3.2.2.1, there is one (fairly rare) case
where fragmented content can be acquired as a single file without any
HAS awareness and that is when fragmented content is used and where
the Manifest File specifies byte range request. This section
discusses how to perform single file acquisition in the other (very
common) cases. To do so, the dCDN would have to have full-HAS
awareness (at least to the extent of being able to map between single
file and individual chunks to serve).
Effect on CDNI interfaces:
o CDNI Metadata Interface: Add fields for indicating the particular
type of HAS (e.g. MPEG DASH or HLS) that is used and whether
segments or fragments are used
o CDNI Metadata Interface: Add field for indicating the name and
type of the Manifest File(s)
Advantages/Drawbacks:
+ Allows for more efficient content acquisition in all HAS-specific
supported forms
- Requires full HAS awareness on part of dCDN
- Requires significant CDNI Metadata Interface extensions
3.2.3. Recommendation
Based on the listed pros and cons, the authors recommend the WG to go
for Option 2.1 since it is sufficient to 'make HAS work'. While
Option 2.2 would bring benefits to the acquisition of large Content
Collections, it would require significant CDNI extensions which would
impact the WG's milestones. Option 2.2 might be a candidate to
include in possible rechartering once the initial CDNI solution is
completed.
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Internet-Draft HTTP Adaptive streaming and CDNI April 20133.3. Request Routing of HAS content3.3.1. General remarks
In this section the effect HAS content has on request routing will be
identified. Of particular interest in this case are the different
types of Manifest Files that might be used. In Section 2.2, three
different methods for identifying and addressing chunks from within a
Manifest File were described: Relative URLs, Absolute URLs without
Redirection and Absolute URLs with Redirection. Of course not every
current CDN will use and/or support all three methods. Some CDNs may
only use one of the three methods, while others may support two or
all three.
An important factor in deciding which chunk addressing method is used
is the Content Provider. Some Content Providers may have a strong
preference for a particular method and deliver the Manifest Files to
the CDN in a particular way. Depending on the CDN and the agreement
it has with the Content Provider, a CDN may either host the Manifest
Files as they were created by the Content Provider, or modify the
Manifest File to adapt it to its particular architecture (e.g. by
changing relative URLs to Absolute URLs which point to the CDN
Request Routing function).
3.3.2. Candidate approaches3.3.2.1. Option 3.1: No HAS awareness
This first option assumes no HAS awareness in both the involved CDNs
and the CDNI Interfaces. This scenario also assumes that neither the
dCDN nor the uCDN have the ability to actively manipulate Manifest
Files. As was also discussed with regards to file management and
content acquisition, having no HAS awareness means that each file
constituting a Content Collections is handled on an individual basis,
with the dCDN unaware of any relationship between files.
The only chunk addressing method that works without question in this
case is Absolute URLs with Redirection. In other words, the Content
Provider that ingested the content into the uCDN created a Manifest
File with each chunk location pointing to the Request Routing
function of the uCDN. Alternatively, the Content Provider may have
ingested the Manifest File containing relative URLs and the uCDN
ingestion function has translated these to Absolute URLs pointing to
the Request Routing function.
In this Absolute URL with Redirection case, the uCDN can simply have
the Manifest File be delivered by the dCDN as if it were a regular
file. Once the client parses the Manifest File, it will request any
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subsequent chunks from the uCDN Request Routing function. That
function can then decide to outsource the delivery of that chunk to
the dCDN. Depending on whether HTTP-based (recursive or iterative)
or DNS-based request routing is used, the uCDN Request Routing
function will then either directly or indirectly redirect the client
to the Request Routing function of the dCDN (assuming it does not
have the necessary information to redirect the client directly to a
surrogate in the dCDN).
The drawback of this method is that it creates a large amount of
request routing overhead for both the uCDN and dCDN. For each chunk
the full inter-CDN Request Routing process is invoked (which can
result in two HTTP redirections in the case of iterative redirection,
or result in one HTTP redirection plus one CDNI Request Routing/
Redirection Interface request/response). Even in the case where DNS-
based redirection is used, there might be significant overhead
involved since both the dCDN and uCDN Request Routing function might
have to perform database lookups and query each other. While with
DNS this overhead might be reduced by using DNS' inherent caching
mechanism, this will have significant impact on the accuracy of the
redirect.
With no HAS awareness, Relative URLs might or might not work
depending on the type of Relative URL that is used. When a uCDN
delegates the delivery of a Manifest File containing Relative URLs to
a dCDN, the client goes directly to the dCDN surrogate from which it
has received the Manifest File for every subsequent chunk. As long
as the Relative URL is not path-absolute (see [RFC3986]), this
approach will work fine.
Since using Absolute URLs without Redirection inherently require a
HAS aware CDN, they also cannot be used in this case. The reason for
this is that with Absolute URLs without Redirection, the URLs in the
Manifest File will point directly to a surrogate in the uCDN. Since
this scenario assumes no HAS awareness on the part of the dCDN or
uCDN, it is impossible for either of these CDNs to rewrite the
Manifest File and thus allow the client to either go to a surrogate
in the dCDN or to a request routing function.
Effect on CDNI interfaces:
o None
Advantages/Drawbacks:
+ Supports Absolute URLs with Redirection
+ Supports Relative URLs
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+ Does not require HAS awareness and/or changes to the CDNI
Interfaces
- Not possible to use Absolute URLs without Redirection
- Creates significant signaling overhead in case Absolute URLs with
Redirection are used (inter-CDN request redirection for each
chunk)
3.3.2.2. Option 3.2: Manifest File rewriting by uCDN
While Option 3.1 does allow for Absolute URLs with Redirection to be
used, it does so in a way that creates a high-level of request
routing overhead for both the dCDN and the uCDN. This option
presents a solution to significantly reduce this overhead.
In this scenario, the uCDN is able to rewrite the Manifest File (or
generate a new one) to be able to remove itself from the request
routing chain for chunks being referenced in the Manifest File. As
described in Section 3.3.2.1, in the case of no HAS awareness the
client will go to the uCDN request routing function for each chunk
request. This request routing function can then redirect the client
to the dCDN request routing function. By rewriting the Manifest File
(or generating a new one), the uCDN is able to remove this first
step, and have the Manifest File point directly to the dCDN request
routing function.
A key advantage of this solution is that it does not directly have an
impact on the CDNI Interfaces and is therefore transparent to these
interfaces. It is a CDN-internal function that a uCDN can perform
autonomously by using information configured for regular CDNI
operation or that is received from the dCDN as part of the regular
communication using the CDNI Request Routing/Redirection Interface.
More specifically, in order for the uCDN to rewrite the Manifest
File, the minimum information needed is the location of the dCDN
request routing function (or alternatively the location of the dCDN
delivering surrogate). This information can be available from
configuration or can be derived from the regular CDNI Request Routing
/Redirection Interface. For example, the uCDN may ask the dCDN for
the location of its request routing node (through the CDNI Request
Routing/Redirection Interface) every time a request for a Manifest
File is received and processed by the uCDN request routing function.
The uCDN would then modify the Manifest File and deliver the Manifest
File to the client. One advantage of this method is that it
maximizes efficiency and flexibility by allowing the dCDN to
optionally respond with the locations of its surrogates instead of
the location of its request routing function (and effectively turning
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the URLs into Absolute URLs without Redirection). There are many
variations around this approach, such as where the modification of
the Manifest File in only performed once (or once per period of time)
by the uCDN request routing function, when the first client for that
particular Content Collection (and redirected to that particular
dCDN) sends a Manifest File request. The advantage of such a
variation is that the uCDN only has to modify the Manifest File once
(or once per time period). The drawback of this variation is that
the dCDN is no longer in a position to influence the request routing
decision across individual content requests.
It should be noted that there are a number of things to take into
account when changing a Manifest File (see for example Section 2.3
and Section 2.4 on live HAS content and ad insertion). Furthermore,
some Content Providers might have issues with a CDN changing Manifest
Files. However, in this option the Manifest File manipulation is
only being performed by the uCDN, which can be expected to be aware
of these limitations if it wants to perform Manifest File
manipulation since it is in its own best interest that its customer's
content gets delivered in the proper way and since there is a direct
commercial and technical relationship between the uCDN (the
Authoritative CDN in this scenario) and its customer (the Content
Provider). Should the Content Provider want to limit Manifest File
manipulation, it can simply arrange this with the uCDN bilaterally.
Effect on CDNI interfaces:
o None
Advantages/Drawbacks:
+ Possible to significantly decrease signaling overhead when using
Absolute URLs.
+ (Optional) Possible to have uCDN rewrite the Manifest File with
locations of surrogates in dCDN (turning Absolute URLs with
Redirection in Absolute URLs without Redirection)
+ No changes to CDNI Interfaces
+ Does not require HAS awareness in dCDN
- Requires high level of HAS awareness in uCDN (for modifying
Manifest Files)
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One of the possibilities with Option 3.2 is allowing the dCDN to
provide the locations of a specific surrogate to the uCDN, so that
the uCDN can fit the Manifest File with Absolute URLs without
Redirection and the client can request chunks directly from a dCDN
surrogate. However, some dCDNs might not be willing to provide this
information to the uCDN. In that case they can only provide the uCDN
with the location of their request routing function and thereby
preventing use of Absolute URLs without Redirection.
One method for solving this limitation is allowing two-step Manifest
File manipulation. In the first step the uCDN would perform its own
modification, and place the locations of the dCDN request routing
function in the Manifest File. Then, once a request for the Manifest
File comes in at the dCDN request routing function, it would perform
a second modification in which it replaces the URLs in the Manifest
Files with the URLs of its surrogates. This way the dCDN can still
profit from having limited request routing traffic, while not having
to share sensitive surrogate information with the uCDN.
The downside of this approach is that it not only assumes HAS
awareness in the dCDN but it also requires some HAS-specific
additions to the CDNI Metadata Interface. In order for the dCDN to
be able to change the Manifest File, it has to have some information
about the structure of the content. Specifically, it needs to have
information about which chunks make up the Content Collection.
Effect on CDNI interfaces (apart from those already listed under
Option 3.2):
o CDNI Metadata Interface: Add necessary fields for conveying HAS
specific information (e.g. the files that make up the Content
Collection) to the dCDN.
o CDNI Metadata Interface: Allow dCDN to modify Manifest File
Advantages/Drawbacks (apart from those already listed under Option
3.2):
+ Allows dCDN to use Absolute URLs without Redirection without
having to convey sensitive information to the uCDN
- Requires high level of HAS awareness in dCDN (for modifying
Manifest Files)
- Requires adding HAS-specific and Manifest File manipulation
specific information to the CDNI Metadata Interface
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Based on the listed pros and cons, the authors recommend to go for
Option 3.1, with Option 3.2 as an optional feature that may be
supported as a CDN-internal behavior by an uCDN. While Option 3.1
allows for HAS content to be delivered using the CDNI interfaces, it
does so with some limitations regarding supported Manifest Files and,
in some cases, with large signaling overhead. Option 3.2 can solve
most of these limitations and presents a significant reduction of the
request routing overhead. Since Option 3.2 does not require any
changes to the CDNI interfaces but only changes the way the uCDN uses
the existing interfaces, supporting it is not expected to result in a
significant delay of the WG's milestones. The authors recommend the
WG to not include Option 3.3, since it raises some questions of
potential brittleness and including it would result in a significant
delay of the WG's milestones.
3.4. Logging3.4.1. General remarks
As stated in [RFC6707], "the CDNI Logging interface enables details
of logs or events to be exchanged between interconnected CDNs".
As discussed in [I-D.draft-bertrand-cdni-logging], the CDNI logging
information can be used for multiple purposes including maintenance/
debugging by uCDN, accounting (e.g. in view of billing or
settlement), reporting and management of end-user experience (e.g.
to the CSP), analytics (e.g. by the CSP) and control of content
distribution policy enforcement (e.g. by the CSP).
The key consideration for HAS with respect to logging is the
potential increase of the number of Log records by two to three
orders of magnitude, as compared to regular HTTP delivery of a video,
since, by default, log records would typically be generated on a per-
chunk-delivery basis instead of per-content-item-delivery basis.
This impacts the scale of every processing step in the Logging
Process (see [I-D.draft-bertrand-cdni-logging]), including:
a. Logging information generation and storing on CDN elements
(Surrogate, Request Routers, ...)
b. Logging information aggregation within a CDN
c. Logging information manipulation (including information
protection, filtering, update and rectification)
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d. (Where needed) Logging information CDNI reformatting (e.g.
reformatting from CDN-specific format to the CDNI Logging
Interface format for export by dCDN to uCDN)
e. Logging exchange via CDNI Logging Interface
f. (Where needed) Logging re-reformatting (e.g. reformatting from
CDNI Logging Interface format into log-consuming specific
application)
g. Logging consumption/processing (e.g. feed logs into uCDN
accounting application, feed logs into uCDN reporting system to
provide per CSP views, feed logs into debugging tools)
Note that there may be multiple instances of step [f] and [g] running
in parallel.
While the CDNI Logging Interface is only used to perform step [e], we
note that its format directly affects step [d] and [f] and that its
format also indirectly affects the other steps (for example if the
CDNI Logging Interface requires per-chunk log records, step [a], [b]
and [d] cannot operate on a per-HAS-session basis and they also need
to operate on a per-chunk basis).
This section discusses the main candidate approaches identified for
CDNI in terms of dealing with HAS with respect to Logging.
3.4.2. Candidate Approaches3.4.2.1. Option 4.1: "Do-Nothing" Approach
In this approach nothing is done specifically for HAS so that each
HAS-chunk delivery is considered, for CDNI Logging, as a standalone
content delivery. In particular, a separate log record for each HAS-
chunk delivery is included in the CDNI Logging Interface in step [e]
(as defined in Section 3.4.1). This approach requires that step [a],
[b], [c], [d] and [e] also be performed on a per-chunk basis. This
approach allows [g] to be performed either on a per-chunk basis
(assuming step [f] maintains per-chunk records) or on a more
"summarized" manner such as per-HAS-Session basis (assuming step [f]
summarizes per-chunk records into per-HAS-session records).
Effect on CDNI interfaces:
o None
Advantages/Drawbacks:
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+ No information loss (i.e. all details of each individual chunk
delivery are preserved). While this full level of detail may not
be needed for some Log consuming applications (e.g. billing),
this full level of detail is likely valuable (possibly required)
for some Log consuming applications (e.g. debugging)
+ Easier integration (at least in the short term) into existing
Logging tools since those are all capable of handling per-chunk
records
+ No extension needed on CDNI interfaces
- High volume of logging information to be handled (storing &
processing) at every step of the Logging process from [a] to [g]
(while summarization in step [f] is conceivable, it may be
difficult to achieve in practice without any hints for correlation
in the log records).
An interesting question is whether a dCDN could use the CDNI Logging
interface specified for the "Do-Nothing" approach to report
summarized "per-session" log information in the case where the dCDN
performs such summarization. The high level idea would be that, when
a dCDN performs HAS log summarization for its own purposes anyways,
this dCDN could include, in the CDNI Logging interface, one (or a
few) log entry for a HAS session (instead of one entry per HAS-chunk)
that summarizes the deliveries of many/all HAS-chunk for a session.
However, the authors feel that, when considering the details of this,
this is not achievable without explicit agreement between the uCDN
and dCDN about how to perform/interpret such summarization. For
example, when a HAS session switches between representations, the
uCDN and dCDN would have to agree on things such as:
o whether the session will be represented by a single log entry
(which therefore cannot convey the distribution across
representations) or multiple log entries such as one entry per
contiguous period at a given representation (which therefore would
be generally very difficult to correlate back into a single
session)
o what would the single URI included in the log entry correspond to?
the Manifest/top-level-playlist/next-level-playlist,...
The authors feel that since explicit agreement is needed between uCDN
and dCDN on how to perform/interpret the summarization, then, to this
can only work if specified as part of the CDNI Logging interface and
then effectively boils down to Option 4.4 defined below ("Full HAS
awareness/per-Session-Logs" Approach).
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We note that support by CDNI of a mechanism (independent of HAS)
allowing the customization of the fields to be reported in log
entries by the dCDN to the uCDN would have a mitigation effect on the
HAS logging scaling concerns because it ensures that only the
necessary subset of fields are actually stored, reported and
processed.
3.4.2.2. Option 4.2: "CDNI Metadata Content Collection ID" Approach
In this approach, a "Content Collection IDentifier (CCID)" field is
distributed through the CDNI Metadata Interface and the same CCID
value is associated through the CDNI Metadata interface with every
chunk of the same Content Collection. The CCID value needs to be
such that it allows, in combination with the content URI, to uniquely
identify a Content Collection. When distributed, and CCID logging is
requested from the dCDN, the dCDN Surrogates are to store the CCID
value in the corresponding log entries. The objective of this field
is to facilitate optional summarization of per-chunk records at step
[f] into something along the lines of per-HAS-session logs, at least
for the Log consuming applications that do not require per-chunk
detailed information (for example billing).
We note that, if the downstream CDN happens to have sufficient HAS
awareness to be able to generate a "Session IDentifier (Session-ID)",
optionally including such Session-ID (in addition to the CCID) in the
per-chunk log record would further facilitate optional summarization
performed at step [f]. The Session-ID value to be included in a log
record by the delivering CDN is such that
o different per-chunk log records with the same Session-ID value
must correspond to the same user session (i.e delivery of same
content to same enduser at a given point in time).
o log records for different chunks of the same user session (i.e
delivery of same content to same enduser at a given point in time)
should be provided with the same session-ID value. While
undesirable, there may be situations where the delivering CDN uses
more than one session-ID value for different per-chunk log records
of a given session, for example in scenarios of fail-over or load-
balancing across multiple Surrogates and where the delivering CDN
does not implement mechanisms to synchronize session-IDs across
Surrogates.
Effect on CDNI interfaces:
o CDNI Metadata interface: One additional metadata field (CCID) in
CDNI Metadata Interface. We note that a similar Content
Collection ID is discussed for handling of other aspects of HAS
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and observe that further thought is needed to determine whether
such CCID should be shared for multiple purposes or should be
independent.
o CDNI Logging interface: Two additional fields (CCID and Session-
ID) in CDNI Logging records.
Advantages/Drawbacks:
+ No information loss (i.e. all details of each individual chunk
delivery are preserved). While this full level of detail may not
be needed for some Log consuming applications (e.g. billing),
this full level of detail is likely valuable (possibly required)
for some Log consuming applications (e.g. debugging)
+ Easier integration (at least in the short term) into existing
Logging tools since those are all capable of handling per-chunk
records
+ Very minor extension to CDNI interfaces needed
+ Facilitated summarization of records related to a HAS session in
step [f] and therefore ability to operate on lower volume of
logging information in step [g] by log consuming applications that
do not need per-chunk record details (e.g. billing) or that need
per-session information (e.g. analytics)
- High volume of logging information to be handled (storing &
processing) at every step of the Logging process from [a] to [f].
3.4.2.3. Option 4.3: "CDNI Logging Interface Compression" Approach
In this approach, a loss-less compression technique is applied to the
sets of Logging records (e.g. Logging files) for transfer on the
IETF CDNI Logging Interface. The objective of this approach is to
reduce the volume of information to be stored and transferred in step
[e].
Effect on CDNI interfaces:
o One additional compression mechanism to be included in the CDNI
Logging Interface
Advantages/Drawbacks:
+ No information loss (i.e. all details of each individual chunk
delivery are preserved). While this full level of detail may not
be needed for some Log consuming applications (e.g. billing),
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this full level of detail is likely valuable (possibly required)
for some Log consuming applications (e.g. debugging)
+ Easier integration (at least in the short term) into existing
Logging tools since those are all capable of handling per-chunk
records
+ Small extension to CDNI interfaces needed
+ Reduced volume of logging information in step [e]
+ Compression likely to be also applicable to logs for non-HAS
content
- High volume of logging information to be handled (storing &
processing) at every step of the Logging process from [a] to [g],
except [e].
3.4.2.4. Option 4.4: "Full HAS awareness/per-Session-Logs" Approach
In this approach, HAS-awareness is assumed across the CDNs
interconnected via CDNI and the necessary information to describe the
HAS relationship across all chunks of the same Content Collection is
distributed through the CDNI Metadata Interface. In this approach,
the dCDN Surrogates leverage the HAS information distributed through
the CDNI metadata and their HAS-awareness to generate summarized
logging information in the very first place (or alternatively, if
per-chunk-logs are generated, to accurately correlate and summarize
per-chunk-logs into per-session logs) for exchange over the CDNI
Logging interface. The objective of that approach is to operate on
lower volume of logging information as early as possible in the
successive steps of the Logging process.
Effect on CDNI interfaces:
o CDNI Metadata Interface: significant extension to convey HAS
relationship across chunks of a Content Collection. Note that
this extension requires specific support for every HAS-protocol to
be supported over the CDNI mesh
o CDNI Logging Interface: extension to specify summarized per-
session logs
Advantages/Drawbacks:
+ Lower volume of logging information to be handled (storing &
processing) at every step of the Logging process from [a] to [g]
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+ Accurate generation of summarized logs because of HAS awareness in
dCDN (for example, where the Surrogate is also serving the
Manifest File(s) for a content collection, the Surrogate may be
able to extract definitive information about the relationship
between all chunks)
- Very significant extensions to CDNI interfaces needed including
per HAS-protocol specific support
- Very significant additional requirement for HAS awareness on dCDN
and for this HAS-awareness to be consistent with the defined CDNI
Logging summarization
- Some information loss (i.e. all details of each individual chunk
delivery are not preserved). The actual information loss depends
on the summarization approach selected (typically the lower the
information loss, the lower the summarization gain) so the right
sweet-spot would have to be selected. While full level of detail
may not be needed for some Log consuming applications (e.g.
billing), the full level of detail is likely valuable (possibly
required) for some Log consuming applications (e.g. debugging)
- Less easy integration (at least in the short term) into existing
Logging tools since those are all capable of handling per-chunk
records and may not be capable of handling CDNI summarized records
- Challenges in defining behavior (and achieving summarization gain)
in the presence of load-balancing of a given HAS-session across
multiple Surrogates (in same or different dCDN)
3.4.3. Recommendation
Because of its benefits (in particular simplicity, universal support
by CDNs and support by all log-consuming applications), the authors
recommend that the per-chunk logging of Option 4.1 be supported by
the CDNI Logging interface as a "High Priority" (as defined in
[I-D.draft-ietf-cdni-requirements]) and be a mandatory capability of
CDNs implementing CDNI.
Because of its very low complexity and its benefits in facilitating
some useful scenarios (e.g. per-session analytics), we recommend
that the CCID mechanisms and Session-ID mechanism of Option 4.2 be
supported by the CDNI Metadata interface and the CDNI Logging
interface as a "Medium Priority" (as defined in
[I-D.draft-ietf-cdni-requirements]) and be an optional capability of
CDNs implementing CDNI.
The authors also recommend that:
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(i) the ability for the uCDN to request that the CCID and Session-ID
field be included in log entries provided by the dCDN be supported
by the relevant CDNI interfaces, and
(ii) the ability for the dCDN to include the CCID field and Session-
ID in CDNI log entries (when the dCDN is capable of doing so) and
indicate so inside the CDNI Logging interface (in line with the
"customizable" log format expected to be defined independently of
HAS),
be supported as a "Medium Priority" (as defined in
[I-D.draft-ietf-cdni-requirements]) and be an optional capability of
CDNs implementing CDNI.
When performing dCDN selection, an uCDN may want to take into account
whether a given dCDN is capable of reporting the CCID and Session-ID.
Thus, the authors recommend that the ability for a dCDN to advertise
its support of the optional CCID and Session-ID capability be
supported by the CDNI request Routing /Footprint and Capabilities
Advertisment Interface as a "Medium Priority" (as defined in
[I-D.draft-ietf-cdni-requirements]).
The authors also recommend that a generic mechanism (independent of
HAS) be supported allowing the customization of the fields to be
reported in logs by CDNs over the CDNI Logging Interface because of
the reduction of the logging information volume exchanged across CDNs
by removing the information that is not of interest to the other CDN.
Because it can be achieved with very little complexity and it
provides some clear storage/communication compression benefits, the
authors recommend that, in line with the concept of Option 4.3, some
existing very common compression techniques (e.g. gzip) be supported
by the CDNI Logging interface as a "Medium Priority" (as defined in
[I-D.draft-ietf-cdni-requirements]) and be an optional capability of
CDNs implementing CDNI.
Because of its complexity, the time it would take to understand the
trade-offs of candidate summarization approaches and the time it
would take to specify the corresponding support in the CDNI Logging
interface, the authors recommend that the log summarization discussed
in option 4.4 not be supported by the CDNI Logging interface at this
stage and be kept as a candidate topic of great interest for a
rechartering of the CDNI WG once the first set of deliverables is
produced. When doing so, we suggest to investigate the notion of
complementing the "push-style" CDNI logging interface supporting
summarization by an on-demand pull-type of interface allowing an uCDN
to request the subset of the detailed logging information that it may
need but is lost in the summarized pushed information.
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The authors note that while a CDN only needs to adhere to the CDNI
Logging interface on its external interfaces and can perform logging
in a different format within the CDN, any possible CDNI Logging
approach effectively places some constraints on the dCDN logging
format. For example, to support the "Do-Nothing" Approach, a CDN
need to perform and retain per chunk logs. As another example, to
support the "Full HAS awareness/per-Session-Logs" Approach, the dCDN
cannot operate on logging format that summarize "more than" or "in an
incompatible way with" the summarization specified for CDNI Logging.
However, the authors feel such constraints are (i) inevitable, (ii)
outweighed by the benefits of a standardized logging interface and
(iii) acceptable because in case of incompatible summarization, all/
most CDNs are capable of reverting to per-chunk logging as per the
Do-Nothing Approach that we recommend as the base mandatory approach.
3.5. URL Signing
URL Signing is an authorization method for content delivery. This is
based on embedding the HTTP URL with information that can be
validated to ensure the request has legitimate access to the content.
There are two parts: 1) parameters that convey authorization
restrictions (e.g. source IP address and time period) and/or
protected URL portion, and 2) message digest that confirms the
integrity of the URL and authenticates the URL creator. The
authorization parameters can be anything agreed upon between the
entity that creates the URL and the entity that validates the URL. A
key is used to generate the message digest (i.e. sign the URL) and
validate the message digest. The two functions may or may not use
the same key.
There are two types of keys used for URL Signing: asymmetric keys and
symmetric key. Asymmetric keys always have a key pair made up of a
public key and private key. The private key and public key are used
for signing and validating the URL, respectively. A symmetric key is
the same key that is used for both functions. Regardless of the type
of key, the entity that validates the URL has to obtain the key.
Distribution for the symmetric key requires security to prevent
others from taking it. Public key can be distributed freely while
private key is kept by the URL signer. The method for key
distribution is out of scope.
URL Signing operates in the following way. A signed URL is provided
by the content owner (i.e. URL signer) to the user during website
navigation. When the user selects the URL, the HTTP request is sent
to the CDN which validates that URL before delivering the content.
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The authorization lifetime for URL Signing is affected by HAS. The
expiration time in the authorization parameters of URL Signing limits
the period that the content referenced by the URL can be accessed.
This works for URL that directly access the media content. But for
HAS content, the Manifest File contains another layer of URL that
reference the chunks. The chunk URL that is embedded in the content
may be requested at an indeterminate amount of time later. The time
period between access to the Manifest File and chunk retrieval may
vary significantly. The type of content (i.e. Live or VoD) impacts
the time variance as well. HAS content has this property that needs
to be addressed for URL Signing.
3.5.2. CDNI Considerations
For CDNI, the two types of request routing are DNS-based and HTTP-
based. The use of symmetric vs. asymmetric key for URL Signing has
implications on the trust model between CSP and CDNs and the key
distribution method that can be used.
DNS-based request routing does not change the URL. In the case of
symmetric key, the CSP and the Authoritative CDN have a business
relationship that allows them to share a key (or multiple keys) for
URL Signing. When the user request a content from the Authoritative
CDN, the URL is signed by the CSP. The Authoritative CDN (as a
Upstream CDN) redirects the request to a Downstream CDN via DNS.
There may be more than one level of redirection to reach the
Delivering CDN. The user would obtain the IP address from DNS and
send the HTTP request to the Delivering CDN, which needs to validate
the URL. This requires the key to be distributed from Authoritative
CDN to the Delivering CDN. This may be problematic when the key is
exposed to the Delivering CDN that does not have relationship with
the CSP. The combination of DNS-based request routing and symmetric
key function is a generic issue for URL Signing and not specific to
HAS content. In the case of asymmetric keys, CSP signs URL with its
private key. The Delivering CDN validates the URL with the
associated public key.
HTTP request routing changes the URL during redirection procedure.
In the case of symmetric key, CSP signs the original URL with the
same key used by the Authoritative CDN to validate the URL. The
Authoritative CDN (as a Upstream CDN) redirects the request to the
Downstream CDN. The new URL is signed by the Upstream CDN with the
same key used by the Downstream CDN to validate that URL. The key
used by the Upstream CDN to validate the original URL is expect to be
different than the key used to sign the new URL. In the case of
asymmetric keys, CSP signs the original URL with its private key.
Authoritative CDN validates that URL with the CSP's public key. The
Authoritative CDN redirects the request to the Downstream CDN. The
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new URL is signed by the Upstream CDN with its private key. The
Downstream CDN validates that URL with the Upstream CDN's public key.
There may be more than one level of redirection to reach the
Delivering CDN. The URL Signing operation described previously
applies at each level between the Upstream CDN and Downstream CDN for
both the symmetric key and asymmetric keys cases.
URL Signing requires support in most of the CDNI Interfaces. The
CDNI Metadata interface should specify the content that is subject to
URL signing and provide information to perform the function. The
Downstream CDN should inform the Upstream CDN that it supports URL
Signing in the asynchronous capabilities information advertisement as
part of the Request Routing interface. This allows the CDN selection
function in request routing to choose the Downstream CDN with URL
signing capability when the CDNI metadata of the content requires
this authorization method. The Logging interface provides
information on the authorization method (e.g. URL Signing) and
related authorization parameters used for content delivery. Having
the information in the URL is not sufficient to know that the
surrogate enforced the authorization. URL Signing has no impact on
the Control interface.
3.5.3. Option 5.1: Do Nothing
"Do Nothing" approach means that CSP can only perform URL Signing for
the top level Manifest File. The top level Manifest File contains
chunk URLs or lower level Manifest File URLs, which are not modified
(i.e. no URL Signing for the embedded URLs). In essence, the lower
level Manifest Files and chunks are delivered without content access
authorization.
Effect on CDNI interfaces:
o None
Advantages/Drawbacks:
+ Top level Manifest File access is protected
+ Upstream CDN and Downstream CDN do not need to be aware of HAS
content
- Lower level Manifest Files and chunks are not protected, making
this approach unqualified for content access authorization
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In addition to URL Signing for the top level Manifest File, CSP
performs flexible URL Signing for the lower level Manifest Files and
chunks. For each HAS session, the top level Manifest File contains
signed chunk URLs or signed lower level Manifest File URLs for the
specific session. The lower level Manifest File contains session-
based signed chunk URLs. CSP generates the Manifest Files
dynamically for the session. The chunk (segment/fragment) is
delivered with content access authorization using flexible URL
Signing which protects the invariant portion of the URL. Segment URL
(e.g. HLS) is individually signed for the invariant URL portion
(Relative URL) or the entire URL (Absolute URL without Redirection)
in the Manifest File. Fragment URL (e.g. Smooth Streaming) is
signed for the invariant portion of the template URL in the Manifest
File. More details are provided later in this section. The URL
Signing expiration time for the chunk needs to be long enough to play
the video. There are implications of signing the URLs in the
Manifest File. For Live content, the Manifest Files are requested at
a high frequency. For VoD content, the Manifest File may be quite
large. URL Signing can add more computational load and delivery
latency in high volume cases.
For HAS content, the Manifest File contains the Relative Locator,
Absolute Locator without Redirection, or Absolute Locator with
Redirection for specifying the chunk location. Signing the chunk URL
requires CSP to know the portion of the URL that remains when the
content is requested from the Delivery CDN surrogate.
For Absolute URL without Redirection, the CSP knows that the chunk
URL which is explicitly linked with the delivery CDN surrogate and
can sign the URL based on that information. Since the entire URL is
set and does not change, the surrogate can validate the URL. The CSP
and the Delivery CDN are expected to have a business relationship in
this case. So either symmetric key or asymmetric keys can be used
for URL Signing.
For Relative URL, the URL of the Manifest File provides the root
location. The method of request routing affects the URL used to
ultimately request the chunk from the Delivery CDN surrogate. For
DNS, the original URL does not change. This allows CSP to sign the
chunk URL based on the Manifest File URL and the Relative URL. For
HTTP, the URL changes during redirection. In this case, CSP does not
know the redirected URL that will be used to request the Manifest
File. This uncertainty makes it impossible to accurately sign the
chunk URLs in the Manifest File. Basically, URL Signing using this
reference method, "as is" for entire URL protection, is not
supported. However, instead of signing the entire URL, the CSP signs
the Relative URL (i.e. invariant portion of the URL) and conveys the
protected portion in the authorization parameters embedded in the
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chunk URL. This approach works the same way as Absolute URL without
Redirection, except the HOST part and (part of) the PATH part of the
URL are not signed and validated. The security level should remain
the same as content access authorization ensures that the user that
requested the content has the credentials. This scheme does not seem
to compromise the authorization model since the resource is still
protected by the authorization parameters and message digest.
Perhaps, further evaluation on security would be helpful.
For Absolute URL with Redirection, the method of request routing
affects the URL used to ultimately request the chunk from the
Delivery CDN surrogate. This case has the same conditions as the
Relative URL. The difference is that the URL is for the chunk
instead of the Manifest File. For DNS, the chunk URL does not change
and can be signed by the CSP. For HTTP, the URL used to deliver the
chunk is unknown to the CSP. In this case, CSP cannot sign the URL
and this method of reference for the chunk is not supported.
Effect on CDNI interfaces:
o Requires the ability to exclude the variant portion of URL in the
signing process (NOTE: Issue is specific to URL Signing support
for HAS content and not CDNI?)
Advantages/Drawbacks:
+ Manifest File and chunks are protected
+ Upstream CDN and Downstream CDN do not need to be aware of HAS
content
+ DNS-based request routing with asymmetric keys and HTTP-based
request routing for Relative URL and Absolute URL without
Redirection works
- CSP has to generate Manifest Files with session-based signed URLs
and becomes involved in content access authorization for every HAS
session
- Manifest Files are not cacheable
- DNS-based request routing with symmetric key may be problematic
due to need for transitive trust between CSP and Delivery CDN
- HTTP-based request routing for Absolute URL with Redirection does
not work because the URL used Delivery CDN surrogate is unknown to
the CSP
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This is similar to the previous section, with the exception that the
Upstream CDN performs flexible URL for the lower level Manifest Files
and chunks. URL Signing for the top level Manifest File is still
provided by the CSP.
Effect on CDNI interfaces:
o Requires the ability to exclude the variant portion of URL in the
signing process (NOTE: Issue is specific to URL Signing support
for HAS content and not CDNI?)
Advantages/Drawbacks:
+ Manifest File and chunks are protected
+ CSP does not need to be involved in content access authorization
for every HAS session
+ Downstream CDN does not need to be aware of HAS content
+ DNS-based request routing with asymmetric keys and HTTP-based
request routing for Relative URL and Absolute URL without
Redirection works
- Upstream CDN has to generate Manifest Files with session-based
signed URLs and becomes involved in content access authorization
for every HAS session
- Manifest Files are not cacheable
- Manifest File needs to be distributed through the uCDN
- DNS-based request routing with symmetric key may be problematic
due to need for transitive trust between uCDN and non-adjacent
Delivery CDN
- HTTP-based request routing for Absolute URL with Redirection does
not work because the URL used Delivery CDN surrogate is unknown to
the uCDN
3.5.6. Option 5.4: Authorization Group ID and HTTP Cookie
Based on the Authorization Group ID metadata, CDN validates the URL
Signing or validates the HTTP cookie for request of content in the
group. CSP performs URL Signing for the top level Manifest File.
The top level Manifest File contains lower level Manifest File URLs
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or chunk URLs. The lower level Manifest Files and chunks are
delivered with content access authorization using HTTP cookie that
contains session state associated with authorization of the top level
Manifest File. The Group ID Metadata is used to associate the
related content (i.e. Manifest Files and chunks). It also specifies
content (e.g. regexp method) that needs to be validated by either
URL Signing or HTTP cookie. Note that the creator of the metadata is
HAS-aware. Duration of the chunk access may be included in the URL
Signing of the top level Manifest File and set in the cookie.
Alternatively, the access control duration could be provided by the
CDNI Metadata interface.
Effect on CDNI interfaces:
o CDNI Metadata Interface - Authorization Group ID metadata
identifies the content that is subject to validation of URL
Signing or validation of HTTP cookie associated with the URL
Signing
o CDNI Logging Interface - Report the authorization method used to
validate the request for content delivery
Advantages/Drawbacks:
+ Manifest File and chunks are protected
+ CDN does not need to be aware of HAS content
+ CSP does not need to change the Manifest Files
- Authorization Group ID metadata is required (i.e. CDNI Metadata
Interface enhancement)
- Requires the use of HTTP cookie which may not be acceptable in
some environments (e.g. where some targeted User-Agents do not
support HTTP Cookie)
- Manifest File has to be delivered by surrogate
3.5.7. Option 5.5: HAS-awareness with HTTP Cookie in CDN
CDN is aware of HAS content and uses URL Signing and HTTP cookie for
content access authorization. URL Signing is fundamentally about
authorizing access to a Content Item or its specific Content
Collections (representations) for a specific user during a time
period with possibly some other criteria. A chunk is an instance of
the sets of chunks referenced by the Manifest File for the Content
Item or its specific Content Collections. This relationship means
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that once the Downstream CDN has authorized the Manifest File, it can
assume that the associated chunks are implicitly authorized. The new
function for the CDN is to link the Manifest File with the chunks for
the HTTP session. This can be accomplished by using an HTTP cookie
for the HAS session.
After validating the URL and detecting that the requested content is
a top level Manifest File, the delivery CDN surrogate sets a HTTP
cookie with a signed session token for the HTTP session. When a
request for a lower level Manifest File or chunk arrives, the
surrogate confirms that the HTTP cookie value contains the correct
session token. If so, the lower level Manifest File or chunk is
delivered due to transitive authorization property. Duration of the
chunk access may be included in the URL Signing of the top level
Manifest File and set in the cookie. The details of the operation
are left to be determined later.
Effect on CDNI interfaces:
o CDNI Metadata Interface - New metadata identifies the content that
is subject to validation of URL Signing and information in the
cookie for the type of HAS content
o Request Routing interface - Downstream CDN should inform the
Upstream CDN that it supports URL Signing for known HAS content
types in the asynchronous capabilities information advertisement.
This allows the CDN selection function in request routing to
choose the appropriate Downstream CDN when the CDNI metadata
identifies the content
o CDNI Logging Interface - Report the authorization method used to
validate the request for content delivery
Advantages/Drawbacks:
+ Manifest File and chunks are protected
+ CSP does not need to change the Manifest Files
- Requires full HAS awareness on part of Upstream CDN and Downstream
CDN
- Requires CDNI Interfaces extensions
- Requires the use of HTTP cookie which may not be acceptable in
some environments (e.g. where some targeted User-Agents do not
support HTTP Cookie)
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- Manifest File has to be delivered by surrogate
3.5.8. Option 5.6: HAS-awareness with Manifest in CDN
CDN is aware of HAS content and uses URL Signing for content access
authorization of Manifest File and chunk. CDN generates or rewrites
the Manifest Files and learns about the chunks based on the Manifest
File. The embedded URLs in the Manifest File are signed by the CDN.
Duration of the chunk access may be included in the URL Signing. The
details of the operation are left to be determined later. Since this
approach is based on signing the URLs in the Manifest File, the
implications for Live and VoD content mentioned in Section 3.5.4
apply.
Effect on CDNI interfaces:
o CDNI Metadata Interface - New metadata identifies the content that
is subject to validation of URL Signing and information in the
cookie for the type of HAS content
o Request Routing interface - Downstream CDN should inform the
Upstream CDN that it supports URL Signing for known HAS content
types in the asynchronous capabilities information advertisement.
This allows the CDN selection function in request routing to
choose the appropriate Downstream CDN when the CDNI metadata
identifies the content
o CDNI Logging Interface - Report the authorization method used to
validate the request for content delivery
Advantages/Drawbacks:
+ Manifest File and chunks are protected
+ CSP does not need to change the Manifest Files
- Requires full HAS awareness on part of Upstream CDN and Downstream
CDN
- Requires CDNI Interfaces extensions
- Requires CDN to generate or rewrite the Manifest File
- Manifest File has to be delivered by surrogate
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The authors consider that Option 5.1 (Do Nothing) is not suitable for
access control of HAS content.
Where the HTTP Cookie mechanism is supported by the targeted User-
Agents and the security requirements can be addressed through proper
use of HTTP Cookies, the authors recommend use of Option 5.4
(Authorization Group ID with HTTP Cookie) and therefore be supported
by the CDNI solution. This method does not require manifest file
manipulation which may be a significant deployment obstacle.
Otherwise, the authors recommend that Option 5.2 (Flexible URL
Signing by the CSP) or Option 5.3 (Flexible URI Signing by the
Upstream CDN) be used and therefore that flexible URI be supported by
the CDNI solution. Option 5.2 and Option 5.3 protect all the
content, does not require Downstream CDN to be aware of HAS, does not
impact CDNI interfaces, supports all different types of devices, and
supports the common cases of request routing for HAS content (i.e.
DNS-based request routing with asymmetric keys and HTTP-based request
routing for Relative URL).
HAS-awareness in CDN (Option 5.5 and Option 5.6) have some advantages
that should be considered for future support (e.g. CDN that is aware
of HAS content can manage the content more efficiently at a broader
context. Content distribution, storage, delivery, deletion, access
authorization, etc. can all benefit.). Including HAS-awareness as
part of the current CDNI charter, however, would almost certainly
delay the CDNI WG's milestones, and the authors therefore do not
recommend it right now.
3.6. Content Purge
At some point in time, a uCDN might want to remove content from a
dCDN. With regular content, this process can be relatively
straightforward; a uCDN will typically send the request for content
removal to the dCDN including a reference to the content which it
wants to remove (e.g. in the form of a URL). Due to the fact that
HAS content consists of large groups of files however, things might
be more complex. Section 3.1 describes a number of different
scenarios for doing file management on these groups of files, while
Section 3.2 list the options for performing Content Acquisition on
these Content Collections. This section will present the options for
requesting a Content Purge for the removal of a Content Collection
from a dCDN.
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The most straightforward way to signal content purge requests is to
just send a single purge request for every file that makes up the
Content Collection. While this method is very simple and does not
require HAS awareness, it obviously creates a signaling overhead
between the uCDN and dCDN since a reference is to be provided for
each content chunk to be purged.
Effect on CDNI interfaces:
o None
Advantages/Drawbacks (apart from those listed under Option 3.3):
+ Does not require changes to the CDNI Interfaces or HAS awareness
- Requires individual purge request for every file making up a
Content Collection (or, alternatively, requires the ability to
convey references to all the chunks making up a Content Collection
inside a purge request) which creates signaling overhead
3.6.2. Option 6.2: Purge Identifiers
There exists a potentially more efficient method for performing
content removal of large numbers of files simultaneously. By
including a "Purge IDentifier (Purge-ID)" in the metadata of a
particular file, it is possible to virtually group together different
files making up a Content Collection. A Purge-ID can take the form
of an arbitrary number or string which is communicated as part of the
CDNI Metadata Interface and which is the same for all files making up
a particular Content Item, and different across different Content
Items. If a uCDN wants to request the dCDN to remove a Content
Collection, it can send a purge request containing this Purge-ID.
The dCDN can then remove all files that share the corresponding
Purge-ID.
The advantage of this method is that it is relatively simple to use
by both the dCDN and uCDN and requiring only limited additions to the
CDNI Metadata Interface and CDNI Control Interface.
The Purge-ID is similar to the Content Collection ID discussed in
Section 3.4.2.2 for handling HAS Logging and we note that further
thought is needed to determine whether the CCID and Purge-ID should
be collapsed into a single element or remain separate elements.
Effect on CDNI interfaces:
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o CDNI Metadata Interface: Add metadata field for indicating Purge-
ID
o CDNI Control Interface: Add functionality to convey a Purge-ID in
purge requests
Advantages/Drawbacks:
+ Allows for efficient purging of content from a dCDN
+ Does not require HAS awareness on part of dCDN
3.6.3. Recommendation
Based on the listed pros and cons, the authors recommend the WG to
have mandatory support Option 1.1, the 'Do Nothing'-approach. In
addition, because of its very low complexity and its benefit in
facilitating low-overhead purge of large numbers of content items
simultaneously, the authors recommend that the Purge IDdentifier of
Option 6.2 be supported as an optional feature by the CDNI Metadata
interface and the CDNI Control interface.
3.7. Other issues
This section includes some HAS-specific issues that came up during
the discussion of this draft and which do not fall under any of the
categories discussed in the previous sections.
- As described in Section 2.2, a Manifest File might either be
delivered by a CDN or by the CSP, thereby being invisible to the
CDN delivering the chunks. Obviously, the decision on whether the
CDN or CSP delivers the Manifest File is made between the uCDN and
CSP, and the dCDN has no choice in the matter. However, some
dCDNs might only want to offer their services in the cases where
they have access to the Manifest File (e.g. because their
internal architecture is based around the knowledge inside the
Manifest File). For these cases, it might be useful to include a
field in the CDNI Capability Advertisement to allow dCDNs to
advertise the fact that they require access to the Manifest File.
4. IANA Considerations
This document makes no request of IANA.
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